The ammeter doesn't care about voltage. It does however, care about amperage. If you're going to step up the voltage you're also probably going to step up the amperage output of the charging system. Pounding a lot of amps through an ammeter made for 30 amps can damage it. I speak from experience. I stayed 6v but put a 6v alternator on my 38. It's a 60 amp alternator. A couple of hard starts later, with the alternator pounding 60 amps through the ammeter trying to re-charge the battery, and my stock ammeter was ruined. I picked up a spare in case my son wants to go back to the generator after I pass and the car becomes his but for now it's on the shelf. I did locate a 6v volt meter so that I can tell if it's charging. Having said all that, yes, if you're going to increase the charging system amperage you'll need to protect the ammeter.

I agree with what Tiny says, but would add a little.

Take notice of the fact that Tiny did NOT convert to 12 volts.

I have several 6 volt vehicles, all of which start and operate just as well as my 12 volt vehicles.

Can we ask WHY you want to convert to 12 volts?

Thanks. Just considering the idea, as I am about to check the electric system. Thank you for you comments about your 6v cars. In relation to amperage, it should go down. This why cables in 12v cars do not need to be as thick as the ones in 6v system (but I may be saying something silly here...).

Posted by: @celso-costa

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 In relation to amperage, it should go down. This why cables in 12v cars do not need to be as thick as the ones in 6v system (but I may be saying something silly here...).

In electrical theory you're dead on. Where the issue may come in is, most people who convert to 12v do so to power accessories. Those can draw considerable amps. Also think back to my personal example. The stock ammeter is designed for a max of 30 amps. If you put a 90 or 120 amp (voltage is irrelevant) alternator on the car, every time you start the car it's going to be pounding that amp load back through an ammeter not designed to handle even close to that load. If you put on a 40 amp alternator it's possible you won't have a problem.